All,
Last fall I asked about how to represent "grid relative" vector components and
the answer was to use the "standard_name" attribute to indicate this. I'm
beginning to wonder whether this will get us into trouble. Here's why;
For u and v velocity components oriented along "i" and "j" directions we specify:
u:standard_name="sea_water_x_velocity"
v:standard_name="sea_water_y_velocity".
Great. That's fine as long as we only have a few of these standard_names, but
what about grid-relative fluxes? We often store fluxes of material in the
NetCDF file because we cannot store the velocity and tracer fields at sufficient
intervals to compute accurate fluxes a posteriori.
The problem is we have *lots* of these. We have many tracer variables and we
therefore have endless grid-relative fluxs with components like "200 micron sand
concentration * sea_water_x_velocity". We don't really want to invent
standard_names for all these, and require that tools like IDV parse the whole
standard_name list just to figure out whether to rotate the vectors or not when
it puts arrows on a map. I think it would be much simpler to have some sort of
property that just indicates "grid_relative". Perhaps via the linkage to a
"grid" variable, as I suggested below?
-Rich
Rich Signell wrote:
> All,
>
> The U and V staggered velocities computed in Arakawa C grid
> models are oriented relative to the grid in the "I" and "J"
> directions, not true east and north (unless the grid happens to
> be oriented N/S, E/W). On a general curvilinear orthogonal grid,
> the orientation of these components change at every grid cell.
>
>
> It seems the proper way to specify this in CF is through the
> use of
>
>
> u:standard_name="sea_water_x_velocity"
> v:standard_name="sea_water_y_velocity",
>
>
> Is this right?
>
>
> One could alternatively have this be inferred for U and V
> variables that have Arakawa C grid defined (via the proposed
> "grid" attribute and "grid_type"="Arakawa C")
>
>
> -Rich
>
Jonathan Gregory wrote:
> Dear Rich
>
>
>>It seems the proper way to specify this in CF is through the use of
>>u:standard_name="sea_water_x_velocity"
>>v:standard_name="sea_water_y_velocity",
>>Is this right?
>
>
> Yes.
>
> The file might also contain northward and eastward velocity components.
> Velocity components relative to lat and lon and relative to the grid are
> different quantities so need different standard names, I think, just like
> northward and eastward components themselves have different names.
>
> Cheers
>
> Jonathan
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>
--
Richard P. Signell
U.S. Geological Survey Phone: (508) 457-2229
384 Woods Hole Road Fax: (508) 457-2310
Woods Hole, MA 02543-1598
Received on Thu May 05 2005 - 07:17:05 BST
